1. Field of the Invention
This invention is concerned with a system for the transmission of DVB/MPEG digital signals, particularly for satellite communication.
2. Description of the Prior Art
Transmission of digital TV multimedia signals using the DVB/MPEG protocol is used not only for point-to-multipoint transmission, such as broadcasting radio or TV programs over a territory, but also for point-to-point transmission, such as radio links, data downloads, IP services. In both cases, the transmission is addressed to users that are scattered over a wide territory, over which the weather conditions may vary in both space and time, thereby inducing changes in the quality of propagation along the satellite link. In order to ensure a satisfactory quality of service (QoS) substantially under all conditions, the ability to correct the errors of the transmission system has to be designed for the worst case, and is consequently turns out to be vastly oversized in the average service conditions, thereby entailing an overall waste of the transponder's power, which can attain the order of 90%.
In the case of radio and television broadcasts, i.e. where the same signal is addressed to all the users in a given territory, such oversizing is unavoidable, because the same signal must be simultaneously received in areas of the territory where the quality of reception is potentially different. However, where the signals are intended for individual users, it would be possible, in principle, to devise a transmission system whose ruggedness is matched to the conditions of propagation at the level of the physical layer, as it is known in other kinds of transmission, by means of the so-called ACM technique (Adaptive Coding and Modulation). In the ACM transmission, the modulator is able to use codings having different ruggedness (QPSK, 8PSK, 16QAM, etc., together with a code having a ratio 1/2, 2/3, 3/4, etc.), whereby the spectral efficiencies decrease with increasing ruggedness; consequently, the ruggedness can be increased on request by the receiver over the return channel, with the penalty of a reduction of the useful bit-rate (see A. Goldsmith, Adaptive modulation and coding for fading channels, Proceedings of the 1999 IEEE, 1999).
The rigidity of the DVB/MPEG protocol has made it impossible in the past to use the adaptive technique mentioned above. More particularly, the standards of this protocol specify that both the chronological order and the rate of the packets be conserved at reception, and that the overall bit-rate of the transport stream be held constant, because these values are used by the receiver for restoring the program synchronization clock: this circumstance, as will be obvious for a person skilled in the art, has been regarded as irreconcilable with the change of bit-rate required for adapting the system to different conditions of transmission.